1. Field of the Invention
This invention relates to a laser scanning apparatus which scans a photosensitive member with laser light, emitted from a light source, deflected by a polygon mirror and reflected by at least one plane mirror.
2. Description of the Prior Art
In the field of laser scanning apparatuses, various contrivances have been conventionally tried in order to make light quantity distribution uniform on a photosensitive member in an image width direction. For example, in a laser scanning apparatus disclosed in JP-A 2001-66527, incident light to a polygon mirror is directed at an angle in an auxiliary scanning direction, while being directed so as to bisect an angular range of deflection in a main scanning direction. With such a construction, since a reflecting angle on the polygon mirror is equal on the left and right sides along the image width direction, it can be avoided to cause a difference in light quantity distribution on the left and right sides along the image width direction on the photosensitive member.
With this construction, however, since incident light impinges on the polygon mirror at an angle in the auxiliary scanning direction, an incidence height relative to the photosensitive member differs slightly in the auxiliary direction according to a light reflecting surface of the polygon mirror if a slightly different error in distance between the rotation axis of the polygon mirror and a light reflecting surface thereof arises between light reflecting surfaces, resulting in pitch non-uniformity on an image. Moreover, with slight tilting of a light reflecting surface of the polygon mirror, an incidence height relative to the photosensitive member is slightly different according to a light reflecting surface of the polygon mirror in the main scanning direction, resulting in a jitter on the image.
Therefore, in the laser scanning apparatus disclosed in JP-A 2002-182143, incident light impinging on a polygon mirror takes a path outside deflecting angles (deflection range). With this construction, incident light impinging on the polygon mirror is directed so as not to have an angle relative to the auxiliary direction, thereby enabling pitch non-uniformity or a jitter on an image to be avoided. Besides, in the laser scanning apparatus, a redirecting mirror (plane mirror) disposed in the optical path after the polygon mirror is applied with coating which makes reflectance values of S polarized light and P polarized light to be almost equal to each other, as an effort for realization of uniformity of a light quantity distribution on a photosensitive member in the image width direction.
In the construction disclosed in JP-A 2002-182143, however, a difference between reflectance values on the polygon mirror (for example, a difference between reflectance values on the polygon mirror corresponding to both ends of an image) is transferred almost in the same way as a difference in light quantity distribution on the left and right sides on the light sensitive member even if reflectance values of S polarized light and P polarized light are made uniform on a plane mirror since no consideration is given to an influence of the polygon mirror. For example, in a scanning optical system having a deflection angle as wide as in excess of 100 degrees, the number of layers of the coating, if coating with which no difference between reflectance values occur on the polygon mirror was applied on the polygon mirror, would be inevitably increased in order to cancel a difference in reflectance because of a wide range of incidence angles of light impinging on the polygon mirror, resulting in cost increase.